Silver powder is widely used in the electronics industry for the manufacture of conductor thick film pastes. These thick film pastes are used to form conductive circuit patterns which are applied to substrates by screen printing. These circuits are then dried and fired to volatilize the liquid organic vehicle and to sinter the silver particles to form the conductor circuit pattern.
Printed circuit technology is requiring denser and more precise electronic circuits. To meet these requirements, the conductive lines have become more narrow in width with smaller distances between lines. The silver powders necessary to form more closely packed, narrower lines must be as close as possible to spherical in shape with narrow particle size distributions.
Many methods currently used to manufacture metal powders can be applied to the production of silver powders. For example, chemical methods, physical processes such as atomization or milling, thermal decomposition, and electro-chemical processes can be used.
Silver powders used in electronic applications are generally manufactured using chemical precipitation processes. Silver powder is produced by chemical reduction in which an aqueous solution of a soluble salt of silver is reacted with an appropriate reducing agent under conditions such that silver powder can be precipitated. The most common silver salt used is silver nitrate. Inorganic reducing agents including hydrazine, sulfite salts, and formate salts can be used to reduce silver nitrate. These processes tend to produce powders which are very coarse in size (greater than 2 microns), are irregularly shaped and have a large particle size distribution due to aggregation.
Organic reducing agents such as alcohols, sugars, or aldehydes are used with alkali hydroxides to create the reducing conditions for silver nitrate. Under these conditions, the reduction reaction is very fast and hard to control and produces a powder with residual alkali ions. Although small in size (&lt;1 micron), these powders tend to have an irregular shape with a wide distribution of particle sizes that do not pack well. These types of silver powders exhibit difficult-to-control sintering and inadequate line resolution in thick film printed conductor circuits.